Electrostatic protection elements are used in semiconductor integrated circuits in order to keep electrostatic discharge (ESD) and electrical overstress (EOS), applied to an input terminal and an output terminal, from exceeding breakdown voltage (VDES) inside the semiconductor integrated circuit.
Mainly used as the electrostatic protection elements are diodes, thyristors, metal-oxide-semiconductor (MOS) transistors, and bipolar transistors.
Having a structure exemplified in FIG. 25, a typical thyristor shows an excellent current driving capability per unit area. Thus, when used for an electrostatic protection element, the thyristor can fit on a small area within the semiconductor integrated circuit. As typical V-I characteristics in FIG. 26 show, however, the thyristor-equipped electrostatic protection element usually has a holding voltage (Vh) lower than a power-supply voltage (VDD) of the semiconductor integrated circuit. Hence, when noise enters the thyristor while the semiconductor integrated circuit is operating; that is when the power-supply voltage is applied to the semiconductor integrated circuit, the thyristor turns on, and a current keeps running between an anode and a cathode. This leads to latchup.
There are two main causes to develop the latchup. The first cause is due to substrate noise (substrate current). The second cause is due to electrical overstress. Both of the causes arise when the latchup continues in the case where the holding voltage (Vh) is lower than the power-supply voltage. In general, the first latchup caused by the substrate noise is defined as withstand current in the current-mode latchup test. The second latchup caused by the electrical overstress is defined as withstand voltage in the supply electrical overstress test.
Once the latchup occurs in the electrostatic protection element, the current will keep running in the electrostatic protection element unless the power supply of the semiconductor integrated circuit is shut down. The overcurrent develops Joule heat, leading to destruction of junction and melted wiring and causing destruction of the electrostatic protection element and the semiconductor integrated circuit.
Thus, for example, Patent Literature 1 discloses a technique to adjust, to increase, the holding voltage of the electrostatic protection element equal to or greater than the power-supply voltage.
FIG. 27 shows a structure of an electrostatic protection element disclosed in Patent Literature 1. An npn transistor forms a thyristor used for an electrostatic protection element 900 of a semiconductor integrated circuit. The npn transistor has (i) a collector region formed of an n-type anode-gate high-concentration impurity region 912, and an n-type well layer 908, (ii) a base region formed of a p-type cathode-gate high-concentration impurity region 918, a p-type well layer 902, and a p-type substrate 901, and (iii) an emitter region formed of an n-type cathode high-concentration impurity region 906. The npn transistor has distance “A” set shorter than usual across a first element isolation insulator 903a. The first element isolation insulator 903a is provided between (i) the p-type cathode-gate high-concentration impurity region 918 included in the base region of the npn transistor and (ii) the n-type cathode high-concentration impurity region 906 included in the emitter region of the npn transistor.
Accordingly, the base-emitter resistance value of the npn transistor decreases, resulting in a successful increase in the voltage for driving the npn transistor as well as in the holding voltage of the electrostatic protection element. This structure makes possible introducing an electrostatic protection element for a semiconductor integrated circuit, freeing the semiconductor integrated circuit from the latchup which leaves a thyristor, forming the electrostatic protection element, on due to an external noise signal.
Hence, in order to prevent the latchup, the thyristor-equipped electrostatic protection element requires (Request 1) operating characteristics showing that the holding voltage (Vh) of the thyristor is higher than the power-supply voltage (VDD).
The electrostatic protection element using the thyristor also requires some other operating characteristics.
In order to achieve the intended purpose of protecting the semiconductor integrated circuit, a trigger voltage (VTRIG); that is a voltage to trigger the operation of the thyristor, shall be equal to or lower than the breakdown voltage (VDES) inside the semiconductor integrated circuit to be protected (Request 2).
FIG. 28 shows an action region constrained by the Requests 1 and 2 in the V-I characteristic graph of the thyristor-equipped electrostatic protection element. The thyristor-equipped electrostatic protection element operates in the whiteout region in FIG. 28 to safely and surely protect the semiconductor integrated circuit to be protected. Such a restricted action region is typically known as a design window for the thyristor-equipped electrostatic protection element.